Prior art in autofocusing principally relies on two techniques. These two techniques and their associated drawbacks will be discussed below.
The most common prior art method of autofocusing an optical system employs the same optics for both the imaging system and the autofocusing system. This type of autofocusing system, typically used in compact disc technology, is depicted in FIG. 1.
The imaging system in its simplest form is a laser diode 4 imaged onto material 3 by lens 1. Lens 1 can be moved by focus actuator 6, typically a coil moving in a magnetic field. Some of the light reflected back from the material 3 is diverted towards the focus detector 5 by an optical element 2, typically a diffraction grating or a partial mirror. As the distance between lens 1 and material 3 changes, a corresponding change is sensed by detector 5, driving actuator 6 via amplifier 7 to null the error out. This is the well known null detector type of focus servo. The principal disadvantage with this system is that the optical imaging system is shared by the main beam and the focus beam, requiring compromises, particularly when laser 4 is used for writing. In this type of autofocus system, the light output of laser 4 is data dependent, making accurate determination of the focus error difficult. This data dependence usually requires a separate, low power, laser diode for focusing. The high power of the write laser can also blind the focus detector, which is attempting to measure the reflected low power beam.
A second prior art autofocus method is depicted in FIG. 2. In this method, a separate distance sensor 8 is used to control actuator 6 in order to keep the distance between lens 1 and material 3 constant. The main drawback with this system is that the control is essentially open loop. Any errors or changes in the position of lens 1 are not sensed by the separate distance sensor 8. Further, if the clearance between sensor 8 and material 3 is large, it is difficult to achieve the required resolution and stability. The system of FIG. 2 does not null out the focus error, it simply moves the lens 1 to the calculated position without any information about whether or not the system is in focus.
Accordingly, it is an object of the present invention to overcome the drawbacks of the prior art by providing a method and apparatus for a null detecting autofocus system.
Another object of the present invention is to employ the autofocusing technique in an optical system wherein the optics used for the autofocus are distinct from those of the primary optical system, so as to eliminate the tradeoffs between focus beams requiring low power and write beams requiring high power which can blind the focus detector.